Tornado warnings 101: What happens when forecasters decide to pull the trigger

An example of tornado warning polygons. Only the areas inside the red box are under a warning in this example, but often outdoor sirens sound throughout the entire affected county. (National Weather Service)

There was a time when the word "tornado" was taboo.

From the late 1800s through the early 20th century meteorologists were not allowed to even mention tornadoes in their forecasts. The U.S. Army Signal Corps banned the word because of the lack of skill in forecasting them and concerns over causing public panic.

In the 1940s things began to change. The Weather Bureau, the forerunner to the National Weather Service, implemented experimental tornado warning systems in several Midwest cities.

Airplanes were thrown about like toys by a tornado on March 20, 1948, at Tinker Air Force Base in Okla., the site of the first official "tornado forecast." (NOAA)

But it took until March 20, 1948, before the very first "tornado forecast" was issued from Tinker Air Force Base in Oklahoma. A tornado later that day hit the base dead-on, causing extensive damage but no fatalities, and the forecast was heralded as a success.

Today tornado warnings are issued fairly often. And while they may be well-understood by the public, the process of issuing them may not be.

Alabama is covered by four National Weather Service offices in Huntsville, Birmingham, Mobile and Tallahassee, Fla. (which covers a portion of southeast Alabama).

(National Weather Service)

Each office has a slightly different process for issuing a tornado warning, but the main points are the same: Identify the threat and get the word out -- fast.

The process starts days in advance. Long-range computer models have gotten increasingly more adept at identifying potential hazards -- sometimes up to a week before the first raindrop falls.

"I see us in a situation now where we have much better lead time on any kind of severe weather outbreak. You know, sometimes we're not even talking about hours or minutes ... on many occasions we're talking about alerting folks to the chance of severe weather days in advance," said JP Dice, the chief meteorologist at Fox 6 in Birmingham. "Forecast modeling has gotten better as computer technology has gotten better. So the models do a pretty good job."

Then it's up to the National Weather Service to make sense of all that data the models serve up.

"Every model has its biases and things you can pick up on," said Kevin Laws, the science and operations officer at the National Weather Service office in Birmingham. "In other words, our job -- just like we do with rain -- is to put some probabilities on the chances of having severe weather. From that, the probability of having a tornado out of a severe weather outbreak."

If the probabilities look good: It's time to start getting the word out.

"Usually when we see a pattern two or three days out depending -- on how far out it is and its predictability -- we start talking with emergency managers and community planners," said Jeffrey Medlin, the meteorologist in charge at the National Weather Service office in Mobile. "We begin aggressively getting the word out with social media, posting things on our website with graphics. It's mainly the 'what, when and where' of the event."

Then it's time to start looking more closely at the details.

A look at just a few of the tornado warnings issued on April 27, 2011, in north and central Alabama. The red boxes are tornado warning polygons. (National Weather Service)

Job No. 1: Identifying what kind of severe weather is on the way -- will it be a cold front with a squall line or will it be supercells?

"That's important -- mode of convection," Laws said. "So we determine the mode of convection. From there we micro in on (the details). So we understand we are going to have supercells. From that are we going to get large hail? Are we going to get tornadoes? Are the tornadoes going to be large?"

"A lot of those supercells that we see develop, most of them don't produce tornadoes," he said. "That's why it's not easy. About 10 percent of supercells produce tornadoes. Sometimes it's less. We have to be the ones being vigilant using our reports coming in from the field."

The process of identifying potential severe weather threats is kind of like baking a cake, Laws said.

There are a lot of players that make up severe weather -- there's no "smoking gun," Laws said.

But two of the big things forecasters monitor are vertical wind shear and instability, Medlin said.

"We monitor carefully the changes in those trends leading in to the event -- the moment when those storms start to develop," he said.

Instability at its most basic is a battle between warm and cold air.

"In other words, how hot it is underneath and how cold it is aloft," Laws said. "The hot rises, the cold air wants to sink. So these two want to change places, so you have an unstable situation where the hot air at the surface is wanting to go up; the cold air above it wants to come down. The two want to switch places. But they can't. So there's tons of struggle and instability there."

How wind shear can lead to the formation of tornadoes. (National Weather Service)

Wind shear is the veering of the wind as you go higher in the atmosphere. It can create the spin that helps spawn tornadoes.

"We micro in on things like wind fields -- and not even necessarily the winds we experience here on the ground," Laws said. "The winds aloft as well are very, very important."

As the window for severe weather draws closer, National Weather Service offices begin getting their people in place for what could be a long day or night.

A peek inside a typical National Weather Service office will find a room filled with computers, monitors and meteorologists -- which Laws refers to as the floor. At his office there's usually one person managing the floor.

"Then you have people working on these environments -- is today still a favorable day for tornadoes?" he said. "You have the radar folks who are interpreting radar, scan to scan. You have folks who are communicating ... if we have a bunch of warnings and they don't get out properly, we're sunk. So we've got to have everybody in the right place."

If all the parameters are coming together a tornado watch may be issued.

Tornado and severe thunderstorm watches are issued by the Storm Prediction Center in Norman, Okla. But when it comes to a warning, that comes directly from the local National Weather Service office.

As storms begin to develop they are watched carefully by weather service radar operators, who are looking for "the strongest rotation, the deepest rotation and the rotation that lasts longest or persists," Medlin said.

"My policy for issuing a warning is this: You want three things," Laws said. "You want a verified report of something happening. You want the radar to be favorable, in other words the rotation to be favorable and high enough to where you consider it to be warning-worthy. And the third ingredient is a favorable environment. I am expecting our folks to have two of those three things in place to issue a tornado warning."

Keep in mind that all this is happening before anything touches the ground in most cases.

"I think people need to understand that this is just a forecast," Laws said. "You can't issue a warning after it is already on the ground -- you need to give lead time."

According to NOAA, the average lead time nationally is 13 minutes. Locally it's even better.

"Whether it's an eight-day forecast or a 15-minute forecast, our goal of lead time is 14 minutes," Laws said. "We need to have a warning out so you can get to your safe place in 14 minutes. If you call me and it's already on the ground, tearing up stuff, what good is it? So I've got to get it out before it actually gets to that point. How challenging is that? Very challenging."

If the parameters are met and it's time to pull the trigger on a warning, then it's up to the software to do the honors.

"The mechanics are the same for every office. There's a certain piece of software that we use (and) it's something that's been a staple in this business for decades now," Laws said. "The wording in the actual warning has changed slightly. As we've grown to learn how people react to warnings, all that stuff has changed, but the physical process of putting out a warning -- the mechanics -- software -- that's pretty much the same. The decision-making process, that's changed a lot."

The software is overlaid on top of the radars, Laws said, so the radar operator then draws the box where he or she wants the warning to go.

(National Weather Service)

That's where polygons come in. In the past, severe weather warnings were issued for entire counties. The polygon warning system was first introduced in October 2007. Its intent is to warn only the locations and people in the direct path of severe weather instead of the entire county, thus cutting down on false alarms.

"I think they've been wonderful," Medlin said. "The No. 1 reason is because there's a tremendous reduction in the warned square area. And I think part of their success has been the Internet and the media partnerships. When there is a warning that's out, there's nothing like having the person on TV zoom in to that polygon and show people right where it is."

Meteorologists are always working to cut down on false alarms and hope to make the warning polygons even smaller in the future.

"We have to remain within the limits of predictability. We don't want to get too cute with it and make it so small and then have it move out of the box," Medlin said. "And sometimes storms, they'll throw us a curveball. Because supercells are known to turn to the right. And sometimes they'll move to the right and that's when the tornado develops. A lot of us, particularly Birmingham and Mobile and Huntsville, will all make the right side of the polygon a little wider knowing that they do that."

Tornadoes can be unpredictable, but National Weather Service meteorologists have had a lot of experience with them, either in Alabama or elsewhere.

"Our warning folks are pretty much top-notch at this point," Laws said. "They've seen it all. They've looked at thousands and thousands and thousands of storms and warnings. Some of them have been storm chasers out in the Midwest. They know how to relate visually what they're seeing in real life to what's happening on the radar, so they have tons of experience with trying to predict with some accuracy -- without a high false alarm rate, I might add -- how to accurately get these things out. Is it perfect? It's not perfect but it's where we are."

The warning is issued, and the word goes out: Weather radios tone in the affected areas, smartphone apps are activated, outdoor sirens begin to blare and TV meteorologists hit the airwaves.

Back at the weather service office, radar operators continue to track storms and update warnings as much as possible with the latest information.

Then it's up to you. What do you do when a tornado warning is issued for where you are?

"This is a re-education campaign. We've transitioned from a false-alarm rate that's traditionally 70-80 percent to one that's now 40," Laws said. "It's the lowest it has ever been historically. And it's probably one of the lowest in the country."

So pay attention the next time you're in the polygon.

"If we issue a tornado warning you better take it seriously," Laws said. "There is a good chance there will be a tornado associated with that storm. A very good chance."